The invention relates to the positioning of a transducer over a magnetic medium and in particular over a data track on a magnetic disk through the detection of previously recorded position or servo information.
Such position information may comprise a so-called "tri-bit" servo pattern including magnetically encoded circular tracks on the magnetic disk having a transducer in close proximity therewith for detecting the information carried by the tracks. Circuitry connected with the transducer provides a position error signal (P.E.S.) of different value depending on the position of the transducer with respect to the magnetic tracks, and this position error signal is supplied to servo actuator and control mechanism for moving the transducer into a particular desired position with respect to the tracks. In this position, the transducer corresponds with a data track that is either detected by this transducer or a transducer mechanically connected with and movable with this transducer, so that the data may be read from the data track (or fresh data may be written on this data track) in this position of the transducer.
The tri-bit servo pattern is disclosed in the IBM Technical Disclosure Bulletin publication of April 1974, pages 3757-3759 and comprises magnetic patterns, portions or segments encoded on a magnetic disk in concentric tracks. The magnetic segments are in pairs or long and short segments, and the ends of long and short segments on alternate tracks are disposed on the same radius or radial line of the disk. The long and short segments are magnetically encoded the same along this radius or radial line and thus provide magnetic flux reversals circularly on each track. A servo transducer positioned over the tracks experiences flux reversals as each junction between magnetic segments passes the transducer gap, and the polarity and amplitude of the resulting pulse vary with the polarity and amplitude of the flux reversal causing them.
The circuitry connected with the servo transducer and producing a position error signal generally includes peak detection circuits for determining the relative amplitudes of the peaks of the signal from the servo tranducer and in one way or another comparing these amplitudes so as to determine whether the servo transducer (and the data transducer if this is a separate transducer) is aligned with the desired data track. In order that the relative amplitudes of the servo peaks may be accurately determined, these peaks should be spaced time-wise with disk rotation so that they do not interfere with each other and so that the leading and trailing ends of the peaks do not overlap adjacent peaks. In order to obtain such spaced pulses, a relatively wide band width is required to accommodate many harmonics of the fundamental frequency detected by the servo transducer; and a relatively slow sampling rate, with a correspondingly small number of magnetic patterns on the disk, is obtained.
The rotatable data storage apparatus disclosed in U.S. Pat. No. 3,936,876 is an example of a magnetic track following servo system utilizing the tri-bit servo pattern. In the structure of this patent, a separate servo transducer effective on the lower side of a magnetic disk is used; and the servo transducer is mechanically coupled with a pair of data transducers effective on the upper surface of the disk. The servo circuitry disclosed in this patent includes a preamplifier receiving the servo signals from the servo transducer and supplying an output to a detector. The detector provides a position error signal the amplitude of which is dependent on the distance that the servo transducer is off of a guide path, and the detector includes a pair of demodulators that are responsive to the peaks of signals supplied from the servo transducer so as to provide the position error signal at the output of the detector. This position error signal is used by means of a compensator and a driver circuit so as to energize a voice coil which moves the servo transducer to correct its position.
Peak detection of a servo transducer output signal has also been used in connection with other servo patterns and in particular with a pattern in which similarly magnetized servo track portions of adjacent tracks are not in edge alignment radially of the disk and in particular in which short and long similarly magnetized servo track portions are centrally disposed radially of the disk. Since peak detection is used, the same disadvantages apply, namely, unduly wide band width with a great number of harmonics required to produce the distinctive signal peaks and a relatively slow sampling rate. One such system is disclosed in U.S. Pat. No. 3,534,344, and the position information is recorded in successive tracks on a separately dedicated disk. Each of the servo tracks contains the spaced set of paired flux reversals, and the sets of flux reversals repeat every other track so that one separately identifiable set of flux reversals will occur in the odd tracks and another separately identifiable set of flux reversals occurs in the even tracks. These are sensed by the servo transducer; and, when the servo transducer is exactly centered over adjacent odd or even tracks, the signal strength from the two adjacent tracks are equal. The peaks of the output signal of the servo transducer caused by adjacent tracks are integrated, and the integrated wave forms are then rectified by half-wave rectifiers and are then applied to peak detectors. Due to the use of these peak detectors, the peaks of the output signal of the transducer must be well spaced as above noted.
The transducer positioning systems of U.S. Pat. Nos. 3,893,180 and 3,959,820 also use the same servo pattern as is used by the apparatus of U.S. Pat. No. 3,534,344 in which similarly magnetized long and short servo track portions of adjacent servo tracks are centrally located with respect to each other. In the system of U.S. Pat. No. 3,893,180, the signal strength of each servo output response is first identified, and the peak amplitudes of the responses are thereafter separately measured and separate signals are produced indicative of the major peak amplitudes. The separate signals are thereafter combined so as to indicate whether the signal strength of either response dominates over the other. The servo responsive system of U.S. Pat. No. 3,959,820 analyzes the output servo signal and defines two separate data tracks for each and every individual track of position information traversed. The circuitry of this patent uses a positive peak detector and a negative peak detector for analyzing the servo transducer output signal. The same disabilities apply to the structures of these two patents, since they both rely on peak detection for determining the position of the servo transducer.